Method and apparatus for shunting induced currents in an electrical lead

ABSTRACT

An electrical lead includes an elongate body having a proximal end portion and a distal end portion, a first electrode disposed adjacent and joined to the distal end portion of the elongate body. Current flow within the first electrode is limited when a predetermined condition occurs, such as the generation of an electromagnetic field having a predetermined frequency range. The medical electrical lead may further comprise one or more second electrodes disposed adjacent the first electrode and joined to the elongate body to shunt current to body tissue when the predetermined condition occurs.

RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.09/999,381 filed Oct. 31, 2001 entitled “Apparatus and Method forShunting Induced Currents in an Electrical Lead”.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for providingelectrical stimuli to tissue or receiving electrical stimulicorresponding to one or more conditions in tissue.

DESCRIPTION OF THE RELATED ART

Since the introduction of the first implantable pacemakers in the 1960s,there have been considerable advancements in both the fields ofelectronics and medicine, such that there is presently a wide assortmentof commercially available body-implantable electronic medical devices.The class of implantable medical devices (IMDs) now includes therapeuticand diagnostic devices, such as pacemakers, cardioverters,defibrillators, neural stimulators, and drug administering devices,among others. Today's state-of-the-art implantable medical devices arevastly more sophisticated and complex than their early counterparts, andare capable of performing significantly more complex tasks. Thetherapeutic benefits of such devices have been well proven.

Modern electrical therapeutic and diagnostic devices for the heartrequire a reliable electrical connection between the device and a regionof the heart. Typically, an electrical contact, commonly referred to asa “lead,” is used for the desired electrical connection. One type ofcommonly used implantable lead is a transvenous lead. Transvenous leadsare generally positioned through the venous system to attach and/orelectrically connect at their distal end via a tip electrode to theheart. At their proximal end, they are typically connected to theelectrical therapeutic and/or diagnostic device, which may be implanted.Such leads normally take the form of a long, flexible, insulatedconductor. Among the many advantages of transvenous leads is that theypermit an electrical contact with the heart without physically exposingthe heart itself, i.e., major thoracic surgery is not required.

Other advancements in medical technology have led to improved imagingtechnologies, for example magnetic resonance imaging (MRI). MRIgenerates cross-sectional images of a human body by using nuclearmagnetic resonance (NMR). The MRI process begins with positioning thebody to be imaged in a strong, uniform magnetic field, which polarizesthe nuclear magnetic moments of protons within hydrogen molecules in thebody by forcing their spins into one of two possible orientations. Thenan appropriately polarized radio-frequency field, applied at resonantfrequency, forces spin transitions between these orientations. The spintransitions create a signal, an NMR phenomenon, which can be detected bya receiving coil.

Further, shortwave diathermy, microwave diathermy, ultrasound diathermy,and the like have been shown to provide therapeutic benefits topatients, such as to relieve pain, stiffness, and muscle spasms; toreduce joint contractures; to reduce swelling and pain after surgery; topromote wound healing; and the like. Generally, energy (e.g., shortwaveenergy, microwave energy, ultrasound energy, or the like) is directedinto a localized area of the patient's body.

Traditionally, however, use of these technologies have been discouragedfor patients having such implanted medical devices, as the environmentproduced by the MRI or diathermy apparatuses is generally consideredhostile to such implantable medical devices. The energy fields,generated during the MRI or diathermy processes, may induce anelectrical current in leads of implantable medical devices. Inconventional leads, the electrical current is typically dissipated viathe lead's tip electrode into tissue adjacent the distal end of thelead. The dissipation of this electrical current may cause resistiveheating in the tissue adjacent the electrode and may result in damage tothe tissue in some cases.

The present invention is directed to overcoming, or at least reducing,the effects of one or more of the problems set forth above.

SUMMARY OF THE INVENTION

In one aspect of the present invention, an electrical lead is presented.The medical electrical lead includes an elongate body having a proximalend portion and a distal end portion, a first electrode disposedadjacent and joined to the distal end portion of the elongate body, anda first conductor extending between the proximal end portion and thedistal end portion of the elongate body and being electrically coupledto the first electrode. The medical electrical lead further comprises asecond electrode disposed adjacent the first electrode and joined to theelongate body and a capacitive device electrically coupled to the firstconductor and the second electrode. The lead further includes acurrent-limiting component within the lead body to limit the flow ofcurrent through the first electrode. Current may be limited upondetection of a predetermined condition, such as the existence of anelectromagnetic field within a predetermined frequency range.

The current-limiting component may include an inductor, an activecircuit component such as a Field Effect Transistor (FET), or aMicro-Electrical-Mechanical system (MEMs) switch. This component may beactivated by a signal generated by a Hall-Effect sensor or anothermagnetic field sensor.

In another aspect of the present invention, a shunting assembly ispresented. The shunting assembly includes an electrode, a conductor, anda capacitive device electrically coupled with the electrode and theconductor. The shunting assembly further comprises a device to limitcurrent within the electrode upon detection of a predeterminedcondition, such as an electromagnetic field within a predeterminedfrequency range.

In another aspect of the present invention, a method is presentedincluding selectively limiting current in a primary current path withina lead body when a predetermined condition is present. During this time,current may be directed via a secondary path to body tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich the leftmost significant digit(s) in the reference numeralsdenote(s) the first figure in which the respective reference numeralsappear, and in which:

FIG. 1 is a stylized view of an embodiment of an implantable medicaldevice according to one embodiment of the present invention, which hasbeen implanted in a human body;

FIG. 2 is a stylized perspective view of an implantable medical devicelead incorporating a shunting assembly according to a first or secondembodiment of the present invention;

FIG. 3 is a schematic diagram of the first embodiment of the shuntingassembly according to the present invention;

FIG. 4 is a schematic diagram of the second embodiment of the shuntingassembly according to the present invention;

FIG. 5 is a stylized perspective view of an implantable medical devicelead incorporating a shunting assembly according to a third embodimentof the present invention;

FIG. 6 is a schematic diagram of the third embodiment of the shuntingassembly according to the present invention;

FIG. 7 is a partial cross-sectional view of an embodiment of theshunting assembly according to the present invention; and

FIG. 8 is a block diagram of a method according to the presentinvention.

FIG. 9 illustrates yet another embodiment of the lead of FIG. 6.

FIG. 10 illustrates an alternative embodiment of the lead of FIG. 6 thatomits the shunting assembly.

FIG. 11 illustrates still another embodiment of the lead of FIG. 6 thatemploys active circuit components to limit current flow within the lead.

FIG. 12 illustrates another embodiment of the lead of FIG. 6 thatemploys Micro-Electrical-Mechanical system (MEMs) switches within thelead.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedeveloper's specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

Embodiments of the present invention concern body-implantable medicaldevices having one or more leads that may be used to stimulate a tissueof a body and/or sense one or more conditions in the tissue. Examples ofsuch implantable medical devices are implantable coronary pacingdevices, pulse generators, defibrillators, neural stimulation devices,electrogram devices, and the like. Generally, these devices operate bymonitoring one or more conditions in the tissue and/or by deliveringelectrical stimuli to the tissue via the lead or leads. For example,such devices may be used to sense cardiac activity, to deliverelectrical pacing stimuli to a portion or portions of a heart, todeliver electrical defibrillating stimuli to a portion or portions ofthe heart, to deliver electrical stimuli to a nerve, to deliverelectrical stimuli to a portion or portions of a nerve bundle, or todeliver electrical stimuli to a portion or portions of a brain. Whilethe description provided herein is directed to an implantable medicaldevice used in a coronary setting, the present invention encompasses anyimplantable medical device, such as those described above, used in anysetting.

FIG. 1 illustrates an embodiment of an implantable medical device 102according to the present invention that has been implanted in a patient104. The implantable medical device 102 includes an implantableelectronic device 106 (e.g., a control unit or the like) housed within ahermetically-sealed, biologically-inert canister 108. The canister 108may itself be conductive so as to serve as an electrode in a circuit ofthe implantable medical device 102. One or more leads 110, 112 areelectrically coupled to the implantable electronic device 106 and extendvia a vein 114 of the patient 104 to a tissue, e.g., a portion of aventricle 116, a portion of an atrium 118, a nerve (not shown), a nervebundle (not shown), or the like. The implantable medical device 102 maybe programmed by using a programming unit 120, which may sendinstructions to and receive information from the implantable medicaldevice 102 via radio-frequency signals.

As shown in FIG. 2, one or more exposed, electrically-conductiveelectrodes, such as a tip electrode 202 or the like, are disposedgenerally near a distal end portion 204 of a body 205 of the lead 110,as well as a distal end of the lead 112 (not shown), if present. Asindicated above, the tip electrode 202 may be used to sense electricalsignals in a tissue, such as in the ventricle 116, in the atrium 118, ina nerve (not shown), in a nerve bundle (not shown), or the like.Further, the tip electrode 202 may be used to deliver electrical stimulito the tissue, such as to deliver electrical stimuli to a portion, orportions, of a heart, to a nerve, or to a portion, or portions, of anerve bundle. The lead 110 further includes a conductor set 206,electrically coupling the implantable electronic device 106, or anelectrical extension (not shown) extending from the implantableelectronic device 106, and one or more electrodes (e.g., the tipelectrode 202 or the like) of the lead 110. Thus, the conductor set 206extends from a proximal end portion (i.e., a portion joinable with theimplantable electronic device 106 or the like) to the distal end portion204 of the body 205.

In a first embodiment, the implantable medical device 102 is a unipolardevice in which the tip electrode 202 may serve as a cathode and thecanister 108 may serve as an anode for pacing, stimulation, or sensingcircuitry (not shown) of the implantable medical device 102. In thisembodiment, as illustrated in FIGS. 2 and 3, a shunting assembly 208includes a ring electrode 302, which is the portion of the shuntingassembly 208 visible in FIG. 2. The conductor set 206 includes a tipconductor 304 that extends through the shunting assembly 208 to the tipelectrode 202. The tip conductor 304 may be a continuous conductor ormay be a plurality of conductors that are electrically interconnected. Acapacitor 306 is electrically coupled between the tip conductor 304 andthe ring electrode 302. The capacitor 306 may take the form of a singlecapacitive device, a plurality of capacitive devices that areelectrically interconnected, or one or more capacitive deviceselectrically interconnected with other electronic devices.

In a second embodiment, as illustrated in FIGS. 2 and 4, the implantablemedical device 102 is a bipolar device in which the tip electrode 202may serve as a cathode for the pacing, stimulation, or sensing circuitry(not shown) of the implantable medical device 102. In this embodiment,the shunting assembly 208 includes a ring electrode 402, which is theportion of the shunting assembly 208 visible in FIG. 2. Further, thering electrode 402 may serve as an anode for the pacing, stimulation, orsensing circuitry of the implantable medical device 102. The conductorset 206 includes a tip conductor 404 that extends through the shuntingassembly 208 to the tip electrode 202. The tip conductor 404 may be acontinuous conductor or may be a plurality of conductors that areelectrically interconnected. The conductor set 206 further includes aring conductor 406 extending into the shunting assembly 208 and to thering electrode 402. As in the tip conductor 404, the ring conductor 406may be a continuous conductor or may be a plurality of conductors thatare electrically interconnected. A capacitor 408 is electrically coupledbetween the tip conductor 404 and the ring electrode 302. The capacitor408 may take the form of a single capacitive device, a plurality ofcapacitive devices that are electrically interconnected, or one or morecapacitive devices electrically interconnected with one or more otherelectronic devices.

In a third embodiment, as illustrated in FIGS. 5 and 6, an implantablemedical device 102 is a bipolar device in which the tip electrode 502may serve as a cathode and a first ring electrode 503 may serve as ananode for the pacing, stimulation, or sensing circuitry (not shown) ofthe implantable medical device 102. In this embodiment, a shuntingassembly 504 includes a second ring electrode 604, which is the portionof the shunting assembly 504 visible in FIG. 5. A conductor set 506includes a tip conductor 606 that extends through the first ringelectrode 503 and the second ring electrode 604 to the tip electrode502. The tip conductor 606 may be a continuous conductor or may be aplurality of conductors that are electrically interconnected. Theconductor set 506 further includes a ring conductor 608 extending to thefirst ring conductor 503. As in the tip conductor 606, the ringconductor 608 may be a continuous conductor or may be a plurality ofconductors that are electrically interconnected. A capacitor 610 iselectrically coupled between the tip conductor 606 and the second ringelectrode 604. The capacitor 610 may take the form of a singlecapacitive device, a plurality of capacitive devices that areelectrically interconnected, or one or more capacitive deviceselectrically interconnected with other electronic devices.

It is often advantageous for patents suffering from certain conditionsto be examined using MRI processes or to be therapeutically treatedusing diathermy processes. However, patients having implantable medicaldevices within their bodies have typically been discouraged fromundergoing such processes, as described above. The present invention, asillustrated in FIGS. 2-6, seeks to reduce this detrimental effect bydissipating induced current in the tip conductor 304, 404, 606 intotissue adjacent the ring electrode 302, 402, 604, as well as into tissueadjacent the tip electrode 202, 502. In this way, the heat, produced bythe dissipating currents, is dispersed over a larger portion of tissue,thus decreasing the likelihood of damage to the tissue.

It is desirable, however, for pacing, stimulation, or sensed signals(e.g., signals of an electrogram or the like) being transmitted over thetip conductor 304, 404, 606, from or to the tip electrode 202, 502, notto be transmitted through the ring electrode 302, 402, 604. Rather, itis desirable for substantially all of such signals to be transmittedbetween the implantable electronic device 106 and the tip electrode 202,502. Accordingly, the capacitors 306, 408, 610 perform filteringfunctions. A high frequency current such as is induced within the leadconductors during MRI or diathermy procedures are routed both to thering electrodes 302, 402, 604, respectively, and the tip electrodes 202,502. However, substantially all of the low-frequency pacing,stimulation, and/or sensed signals traveling over the tip conductors304, 404, 606 are routed only to the tip electrodes 202, 502. For thepurposes of this disclosure, the phrase “substantially all” of thepacing, stimulation, or sensed signals is defined as a level of signalat which the implantable medical device 102 is capable of operatingproperly.

The shunting assembly 208, 504 operates by employing the variableimpedance characteristics of the capacitor 306, 408, 610. Generally,currents induced in conductors (e.g., the tip conductor 304, 404, 606)by energy fields emitted by MRI and diathermy equipment are greater thanabout one megahertz (MHz). Further, signals, such as pacing signals,stimulation signals, sensed signals, and the like, generally havefrequencies of less than about 500 hertz (Hz). According to embodimentsof the present invention, by taking into account the inherent electricalimpedance of tissue of about 500 ohms (Ω), the capacitance of thecapacitor 306, 408, 610 can be determined such that a portion of thecurrent induced in the tip conductor 304, 404, 606 by the MRI ordiathermy equipment is passed through the capacitor 306, 408, 610 to thering electrode 302, 402, 604, while signals, such as pacing signals,stimulation signals, sensing signals, and the like are not passedthrough the capacitor 306, 408, 610, but are rather transmitted over thetip conductor 304, 404, 606 directly to the tip electrode 202, 502. Inother words, the capacitor 306, 408, 610 acts as a filter to only allowcurrents having frequencies within a certain range to be routed to thering electrode 302, 402, 604. In one embodiment, the capacitor 306, 408,610, in combination with the impedance of the tip electrode 202 and thetissue, allows a high-pass filter to be created at certain frequenciessuch as those exceeding 1 MHz.

For example, given MRI-induced currents having a frequency of two MHzand a sensed signal (e.g., an electrogram signal, or the like) of 100Hz, a one nanofarad (nF) capacitor (e.g., the capacitor 306, 408, 610,or the like) has a electrical impedance of about 80 Ω at a frequency ofabout two MHz and has a electrical impedance of about 1.6 megohms (MΩ)at a frequency of about 100 Hz, as demonstrated by the equation:$X_{C} = \frac{1}{2\pi\quad{fc}}$

wherein:

X_(C)=the impedance of the capacitor (Ω);

f=the frequency (Hz); and

c=the capacitance of the capacitor (F).

Thus, in this example, the induced currents would pass through the tipelectrode 202, 502, as well as through the capacitor 306, 408, 610 tothe ring electrode 302, 402, 604, since the electrical impedance of thecapacitor 306, 408, 610 is about 160Ω, which is less than the electricalimpedance of tissue adjacent the tip electrode 202, 502 and the ringelectrode 302, 402, 604 (500 Ω). In this case, the induced currentswould be divided approximately 14 percent (80 Ω/580 Ω) to the tipelectrode 202, 502 and approximately 86 percent (500 Ω/580 Ω) to thering electrode 302, 402, 604. The sensed signal would be substantiallyunaffected, since the electrical impedance of the capacitor 306, 408,610 is about 1.6 MΩ at 100 Hz, thereby providing a high-pass filteringeffect.

In one embodiment, the electrical impedance of the capacitor 306, 408,610 at frequencies typical of the induced current is below aboutone-fifth (about 20 percent) of the impedance of the tissue adjacent thetip electrode 202, 502 and adjacent the ring electrode 302, 402, 604(e.g., 100 Ω in the example). In another embodiment, the electricalimpedance of the capacitor 306, 408, 610 at frequencies typical ofpacing, stimulation, or sensed signals is about ten times the impedanceof the tissue adjacent the tip electrode 202, 502 and adjacent the ringelectrode 302, 402, 604 (e.g., 5000 Ω in the example). Further, bysizing the surface area of the ring electrode 302, 402, 604 to be atleast about three times the surface area of the tip electrode 202, 502,the current density may be reduced by at least about four times, thusleading to a commensurate reduction in temperature rise in the tissueadjacent the tip electrode 202, 502 and the ring electrode 302, 402,604. In one embodiment, the surface area of the tip electrode 202, 502,as discussed herein, refers to the surface area of the tip electrode202, 502 omitting any surface area attributed to microstructural pits,crevices, indentations, or the like that may be conventionally used toincrease the electrical contact area of the tip electrode 202, 502. Suchmicrostructural pits, crevices, indentations, or the like, in oneembodiment, may have diameters of less than about 200 micrometers.

A shunting assembly 702 according to one embodiment of the presentinvention is illustrated in FIG. 7. The shunting assembly 702, whichmay, in one embodiment, be hermetically sealed, includes a tube 704 thatis joined (e.g., by welds 706 or the like) to end caps 708, 710.Capacitors 712, 714 are electrically connected with and joined (e.g., bywelds 716 or the like) to the end caps 708, 710, respectively. In oneembodiment, the capacitors 712, 714 are discoidal capacitors or the likehaving central contacts 711, 713, respectively, and peripheral contacts715, 717, respectively. The shunting assembly 702 further includes pins718, 720 that are interconnected by a central conductor 722 by joints724. The pins 718, 720 are electrically connected with the centralcontacts 711, 713, respectively. Further, the pin 718 is electricallyconnected with a proximal conductor 726 (shown in phantom) of the lead110, which is electrically connectable with the implantable electronicdevice 106. The pin 720 is electrically connected with a distalconductor 728 (shown in phantom) of the lead 110, which is electricallyconnected with the tip electrode 202, 502 (FIGS. 2 and 5). Thus, theproximal conductor 726, the pin 718, the central conductor 722, the pin720, and the distal conductor 728 comprise the tip conductor 304, 404,606 (FIGS. 3, 4, and 6).

The capacitors 712, 714 are selected as described above, such thatsignals having a certain range or ranges of frequencies (i.e., inducedcurrents) may flow both through the tip conductor 304, 404, 606 to thetip electrode 202, 502 and through the tube 704, which serves as thering electrode 302, 402, 604. Signals having another range or ranges offrequencies (i.e., pacing, stimulation, sensed signals, or the like) maysubstantially only flow through the tip conductor 304, 404, 606 to thetip electrode 202, 502, as the capacitors 712, 714 have sufficientimpedance to prevent the signals from flowing therethrough. While twocapacitors 712, 714 are illustrated in FIG. 7, the present inventionencompasses a shunting assembly 702 having one or more capacitors suchas the capacitors 712, 714. Thus, the shunting assembly 702 is oneembodiment of the shunting assembly 208, 504 illustrated in FIGS. 2-6.

A method according to one embodiment of the present invention isillustrated in FIG. 8. In one embodiment, the method includesselectively routing an electrical current traveling through a conductor(e.g., the tip conductor 304, 404, 606 or the like) electrically coupledwith body tissue (e.g., tissue of the patient 104 or the like) over atleast one of a primary path and a secondary path to the body tissuebased upon the characteristic of the electrical current (block 802). Inone embodiment, the primary path may be through the tip conductor 304,404, 606 and the tip electrode 202, 502. Further, the secondary path maybe through the capacitor 306, 408, 610 and the ring electrode 302, 402,604. In one embodiment, the characteristic of the electrical currentcomprises the frequency of the electrical current.

In another embodiment of the present invention, selectively routing theelectrical current, as described above, further comprises routing thecurrent over the primary path and the secondary path to the body tissueif the current is induced in the conductor by a field (block 804). In afurther embodiment, selectively routing the electrical current, asdescribed above, further comprises routing the current only over theprimary path to the body tissue if the current is not induced in theconductor by a field (block 806).

FIG. 9 illustrates yet another embodiment of the lead of FIG. 6. As inFIG. 6, implantable medical device 102 is a bipolar device in which thetip electrode 502 may serve as a cathode and a first ring electrode 503may serve as an anode for the pacing, stimulation, or sensing cardiacsignals. Tip conductor 606 extends to tip electrode 502, and ringconductor 608 extending to first ring conductor 503. A capacitor 610 iselectrically coupled between the tip conductor 606 and second ringelectrode 604. The capacitor 610 may take the form of a singlecapacitive device, a plurality of capacitive devices that areelectrically interconnected, or one or more capacitive deviceselectrically interconnected with other electronic devices. In thisembodiment, second ring electrode 604 is further coupled via a secondcapacitor 902 to first ring electrode 503.

Also shown in FIG. 9 is an optional inductor 904. This inductor providesa high impedance path to the tip electrode 502 when high frequencysignals are induced within conductor 606, as when the lead is subjectedto RF electromagnetic fields. This high-impendence path furtherdecreases the current flowing through the tip, thereby furtherminimizing heating effects at the lead tip. A similar inductor may beincorporated into any of the foregoing embodiments in a similar mannerto that shown in FIG. 9. This inductor may be positioned at any locationalong conductor 606, but is optimally positioned substantially proximateto the tip electrode. In yet another embodiment, the inductor may bepositioned at a more proximal location on conductor 606.

The embodiments discussed above include a shunting assembly. It may benoted, that in another embodiment of the invention, this shuntingassembly is omitted, with only an inductor being used to limit currentflow within one or more conductors of the lead.

FIG. 10 is a side view of a unipolar lead without a shunting assembly.Second ring electrode 604 is omitted, as is ring electrode 503. Inductor904 is provided is relatively close proximity to tip electrode 502. Asnoted above, this inductor attenuates the frequencies of electrodemagnetic signals used for MRI.

The foregoing examples discuss the use of passive components within thelead to minimize tissue injury when the lead is subjected to magneticfields. In another embodiment, active components may be used instead of,or in addition to, passive components to reduce current flow within thelead. For example, a CMOS Field Effect Transistor (FET) may be used as aswitch within a lead body to prevent current flow when the lead isexposed to RF electromagnetic energy.

FIG. 11 illustrates still another embodiment of the lead of FIG. 6. Inthis embodiment, one or more active circuit components 1000 are providedin series with conductor 606. Conductors 606 and 608 are shown coupledto an implantable medical device (IMD) 1002. A control signal 1004 isprovided to one or more active components 1000 when the presence of ahigh-frequency electromagnetic field is detected. This detection may beaccomplished using a frequency-sensitive hall-effective sensor, amagnetoresistive magnetic field sensor, or any other type of sensor 1006that has been adapted to sense the presence of electromagnetic signalswithin a predetermined frequency range. In the illustrated embodiment,the sensor 1006 resides within the housing of the IMD, and the detectionsignal is provided by sensor and associated control circuitry as controlsignal 1004. In another embodiment, the sensor may be included within,or on, the lead body in proximity to the one or more active circuitcomponents 1000, with the control signal being routed between the one ormore active components 1000 and the sensor 1006.

As stated above, the one or more circuit components may include a FET,with the gate of the transistor being coupled to the control signal 1004so that the flow of current through the distal portion of conductor 606may be selectively disabled. This prevents all current from flowingthrough the tip electrode 502 when the lead is placed within ahigh-frequency electromagnetic field. Any other active circuit componentthat may be adapted for use in a switching network and employable tocontrol currents induced by RF electromagnetic fields as discussedherein may be used in the alternative. It may be noted that in anotherembodiment, the lead of FIG. 10 need not include ring electrodes 503 and604. In this instance, tissue heating and subsequent injury at tipelectrode 502 is prevented by opening the switching circuit comprised ofactive circuit components 1000. Finally, it may be noted that activecircuit components may be incorporated into any of the foregoing leadembodiments in a similar manner as shown in FIG. 10.

In yet another embodiment of the invention, Micro-Electrical-Mechanicalsystem (MEMs) switches may be used in place of one or more transistornetworks in a manner similar to that discussed above. For example, FIG.12 illustrates MEMs switching network 1100 coupled to control signal1004 to control current flow in conductor 606. Such switches, which havedimensions in a range of less than 10 microns, can be manufactured onsilicon, as described in U.S. Pat. Nos. 6,070,101, 6,081,748, 6,122,545,and 6,148,234 incorporated herein by reference in their entireties.These switches may be opened or closed using control signals such ascontrol signal 1004. Use of MEMs technology within an implantablemedical device is described in commonly-assigned U.S. Patent Applicationentitled “MEMs Switching Circuit for an Implantable Medical Device”,incorporated herein by reference in its entirety. As discussed above inreference to FIG. 10, the lead of FIG. 11 need not include ringelectrodes 503 and 604. Tissue heating is prevented by opening the MEMsswitching network 1100. MEMs components may be incorporated into any ofthe foregoing lead embodiments in a similar manner as shown in FIG. 11.

While the operation of the present invention has been disclosed relativeto energy fields emitted by MRI and diathermy equipment, the presentinvention is not so limited. Rather, the operation of the presentinvention is equally applied to energy fields emitted by equipment otherthan MRI and diathermy equipment.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. It is therefore evident that the particularembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope and spirit of the invention.Accordingly, the protection sought herein is as set forth in the claimsbelow.

1. An electrical lead, comprising: an elongate body having a proximalend portion and a distal end portion; a first electrode disposed nearthe distal end portion of the elongate body; a first conductorelectrically coupled to the first electrode; a second electrode disposedadjacent the elongate body; a capacitive device electrically coupled tothe first conductor and the second electrode; and a component coupled tothe first conductor to selectively limit current through the firstelectrode.
 2. The lead of claim 1, wherein the component is an inductor.3. The lead of claim 1, wherein the component is a switch.
 4. The leadof claim 3, wherein the component is coupled to a sensor capable ofdetecting an electromagnetic field.
 5. The lead of claim 4, wherein thesensor is capable of detecting an electromagnetic field in apredetermined frequency range.
 6. The lead of claim 5, wherein theswitch includes active circuit components.
 7. The lead of claim 5,wherein the switch includes Field Effect Transistors (FETs).
 8. The leadof claim 5, wherein the switch includes at least oneMicro-Electrical-Mechanical System (MEMs) switch.
 9. The lead accordingto claim 1, further comprising a second conductor extending between theproximal end portion of the elongate body and the second electrode endbeing electrically coupled to the second electrode.
 10. The leadaccording to claim 1, further comprising: a third electrode disposedadjacent the second electrode and joined to the elongate body; and asecond conductor extending between the proximal end portion of theelongate body and the third electrode and being electrically coupled tothe third electrode.
 11. The lead according to claim 1, wherein thecapacitive device further comprises a discoidal capacitor having acentral contact and a peripheral contact; the first conductor iselectrically coupled with the central contact; and the second electrodeis electrically coupled with the peripheral contact.
 12. The leadaccording to claim 1, wherein: the first electrode and the secondelectrode are capable of being electrically coupled with body tissue;the first conductor is capable of transmitting an electrical current;and the capacitive device has an electrical impedance substantially lessthan electrical impedance of the body tissue at signal frequenciesgreater than those used to sense biological signals or deliverelectrical stimulation to the body tissue.
 13. The lead according toclaim 1, wherein: the first electrode and the second electrode arecapable of being electrically coupled with body tissue; the firstconductor is capable of transmitting an electrical current; and thecapacitive device has an electrical impedance substantially greater thanan electrical impedance of the body tissue at signal frequencies used tosense biological signals or deliver electrical stimulation to the bodytissue.
 14. The lead according to claim 1, wherein an area of the secondelectrode is at least three times an area of the first electrode. 15.The lead according to claim 1, wherein the capacitive device is capableof allowing at least a portion of a current, induced in the firstconductor by a field, to be routed to the second electrode.
 16. Amedical electrical lead for an implantable medical device having asensor to sense a predetermined physiological condition, comprising: anelongate body; an electrode; a component coupled to the elongate body tolimit the flow of current through the electrode upon detection of thepredetermined physiological condition.
 17. The lead of claim 16, whereinthe sensor is adapted to sense an electromagnetic field.
 18. The lead ofclaim 17, wherein the component is configured to limit the flow ofcurrent through the electrode upon detection of an electromagnetic fieldin a predetermined frequency range.
 19. The lead according to claim 18,wherein the component is an active circuit component.
 20. The leadaccording to claim 19, wherein the component is a field effecttransistor.
 21. The lead according to claim 18, wherein the component isa Micro Electrical-Mechanical system (MEMs) switch.
 22. The leadaccording to claim 18, and further comprising a shunting assemblycoupled to the elongate body to dissipate heat when the lead is exposedto the electromagnetic field in the predetermined frequency range. 23.The lead according to claim 22, wherein the shunting assembly comprisesa capacitive device.
 24. The lead according to claim 23, wherein thecapacitive device is a discoidal capacitor having a central contact anda peripheral contact, wherein the electrode is electrically coupled withthe peripheral contact, and further comprising a conductor electricallycoupled with the central contact.
 25. A method of utilizing a medicalelectrical lead, wherein the lead includes an elongate body and anelectrode coupled to the elongate body, comprising the steps of: a.)providing a primary current path within the elongate body to theelectrode; b.) limiting current in the primary current path when apredetermined condition is present within the medical electrical lead;c.) providing at least one secondary current path to carry current whenthe predetermined condition is sensed.
 26. The method of claim 25,wherein the predetermined condition of step b.) is the presence in theprimary current path of a current having a frequency within apredetermined frequency range.
 27. The method of claim 26, wherein stepb.) includes limiting current via a inductor placed within the primarycurrent path.
 28. The method of claim 25, wherein step b.) comprisessensing an electromagnetic field, and limiting current when the sensedelectromagnetic field has a magnetic field strength within apredetermined range.
 29. The method of claim 28, wherein step b.)comprises limiting current by opening a switch.
 30. The method of claim29, wherein step b.) comprises opening a MEMs switch.
 31. The method ofclaim 28, wherein step b.) comprises using an active circuit componentto limit current flow.
 32. The method of claim 31, wherein step b.)comprises using an FET to limit current flow.
 33. The method of claim25, and further including: d.) routing current from the primary currentpath to the secondary current path when the predetermined condition ispresent.
 34. The method of claim 33, wherein step d.) includes providinga capacitor between the primary current path and the secondary currentpath to shunt current having a frequency within a predeterminedfrequency range there between.